We investigate the effect of thermal annealing on the optical, morphological and photovoltaic (PV) properties of bulk heterojunction (BHJ) solar cell based on the poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C12) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). The ITO/PEDOT:PSS/PBTTT-C12:PC71BM/Al devices were fabricated on glass substrates from the PBTTT-C12:PC71BM (1:4) solution in dichlorobenzene. Atomic force microscopy (AFM) is used to investigate the surface morphology of the PBTTT-C12:PC71BM thin films. The AFM results show that the thin film’s surface roughness decreases with increasing annealing temperature, making the annealed films smoother as compared to the nonannealed sample. The efficiency of the ITO/PEDOT:PSS/PBTTT-C12:PC71BM/Al PV devices increased from 1.85% to 2.48% with an increase in temperature from 0 °C to 150 °C.